1. Field of the Invention
The present invention relates to a focusing device used in an imaging system.
2. Description of the Related Art
Focusing devices are widely used in imaging systems for improved imaging. Generally, a focusing device includes a stepper motor used for focusing a lens of the imaging system. Without a preceding excitation, a still stepper motor may be rotated incorrectly following an attempt to abruptly travel at high speeds, resulting in a possible failure to properly guide and focus the lens. Similarly, a succeeding excitation is needed to gradually halt the stepper motor. Moreover, the preceding and succeeding excitations to the stepper motor help to ensure the reliability of its rotation.
An auto-focusing approach of the imaging system is that the lens of the imaging system is driven by the stepper motor to different focusing positions according to a vertical-synchronization-signal sequence in order to obtain an auto-focusing curve. A peak of the auto-focusing curve shows the best focusing position of the lens, thereby bringing an image into focus.
The vertical-synchronization-signal sequence includes a plurality of vertical synchronization signals. A time interval between two adjacent vertical synchronization signals forms a vertical-synchronization-signal time. Each vertical-synchronization-signal time may include a sub time and an exposure time (shutter speed) arranged sequentially. The sub time may further include three sequential stages: the preceding excitation is applied to the stepper motor in a first stage, a phase-changed excitation is applied to the stepper motor in a second stage, and the succeeding excitation is applied to the stepper motor in a third stage. However, the stepper motor does not rotate during the first and the third stages, which results in a long duration of focusing.
What is needed, therefore, is to provide a focusing device, in which the above problem is eliminated or at least alleviated.